1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory having a double gate structure.
2. Description of Related Art
In the prior art, this type of semiconductor memory intended for a high integration density has been proposed by K. Sekiya et al in "1986 VLSI SYMPOSIUM" (Digest of Technology Paper, 1986 VLSI SYMPOSIUM, p87). In the proposed semiconductor memory, trenches are formed in a semiconductor substrate by etching, and device isolation regions are formed by filling up the trenches with an insulating material.
This structure is effective in increasing the integration density to a certain extent. However, a margin for an overlapping or alignment between a word line and the device isolation regions has been required so as to ensure that the word line never sticks out or protrudes from the device isolation regions into a source region. This is a hindrance in reducing the scale of the memory array in a widthwise direction of the word line. Particularly, in order to reduce the scale of the memory array in a widthwise direction of the word line, if the source region is reduced, a source resistance is inevitably increased, with the result that a writing characteristics and a read characteristics of memory cells are deteriorated.